The present invention relates to a color developer composition which is designed to be used with recording systems which employ colorless electron donating materials to form images.
Recording materials utilizing developer materials to produce colored images from colorless or substantially colorless electron donating materials are well-known. Specific examples of such recording materials include pressure sensitive carbonless copying paper, heat-sensitive recording paper, electrothermographic recording paper, Cycolor.RTM. photographic materials and the like. They are described in more detail in U.S. Pat. Nos. 2,712,507; 2,730,456; 2,730,457; 3,418,250; 3,432,327; 3,981,821; 3,993,831; 3,996,156; 3,996,405 and 4,000,087. These papers include a developer sheet (also referred to as a CF (coated front) sheet) comprising a substrate coated with an electron acceptor which reacts with a leuco dye transferred to the surface of the developer sheet to form an image thereon.
Much research has been directed to developing new and improved developers for use in the aforementioned recording materials. Representative examples of the developers that have been used include phenol derivatives and phenolic resins, biphenols, methylene bis-diphenols, phenol-formaldehyde novolak resins, metal processed novolak resins, salicylic acid derivatives and salts. See U.S. Pat. No. 3,934,070 to Kimura teaching salicyclic acid derivatives; U.S. Pat. No. 3,244,550 to Farnham teaching biphenols, diphenols, and resinous products containing them, and U.S. Pat. No. 3,244,549 to Farnham teaching phenol derivatives. Representative examples of phenol-formaldehyde condensates previously used in the art are described in numerous patents, including U.S. Pat. No. 3,672,935. Among color developers, phenol-formaldehyde condensates have been widely used because they exhibit excellent color development, good coating properties (rheology) and good water resistance. However, phenolic resins are somewhat colored materials and become even more colored as they are exposed to ambient conditions. Such discoloration is a very undesirable attribute in imaging systems where aesthetic appearance is of extreme importance. Fading of the image when exposed to extreme temperatures and humidity is also an undesirable trait of the currently developer materials. Therefore, it is a principal object of the present invention to provide an improved developer composition for use with recording materials which employ leuco dyes such as carbonless copy paper and photo imaging systems which overcome some of the deficiencies of prior art developers.